In recent years, the search for offshore oil and gas reserves has been carried into water depths considerably deeper than those from which most oil and gas production has been conducted to date. Producing oil and gas from reservoirs in deep water regions presents a host of technical problems. One of the most challenging of these has been the development of deep water platforms from which drilling and production activities can be conducted. Most current drilling and production of offshore hydrocarbons is conducted from platforms consisting of a work deck supported above the ocean surface by a rigid concrete or tubular steel structure which is rigidly secured to the ocean bottom. Such platforms are well suited for a water depths of up to 250-350 meters. As water depths exceed this, it becomes increasingly difficult and expensive to produce a structure which will rigidly resist the wave, wind and current loadings to which it will be exposed. It is generally considered economically impractical to use rigid structures for oil and gas production in water depths beyond about 400 meters.
To avoid the strong depth sensitivities of conventional rigid offshore drilling and production structures a number of alternate structures have been proposed. One such alternate structure is the tension leg platform (TLP). The general configuration of a TLP is illustrated in FIG. 1 of the appended drawings. A TLP has a buoyant hull supporting a work deck from which drilling and producing activities are conducted. The hull is moored to a foundation on the ocean bottom by a set of elongate tethers which are secured to the buoyant hull under tension. The tensioned tethers maintain the hull at a significantly greater draft than it would assume if free floating. The balance of forces imposed by buoyancy and the tensioned tethers limits the degree to which the TLP undergoes motion in response to forces imposed by waves, ocean currents and wind. It has been suggested that TLPs could be employed in water depths up to 3000 meters, whereas the deepest present application of a conventional rigid offshore drilling and production structure is in a water depth of approximately 410 meters.
Though TLPs avoid many of the disadvantages faced by conventional platforms in deep water, they do present their own special problems. One area of TLP design and operation that has proven especially troublesome concerns the system for installing and tensioning the tethers. In most TLP designs proposed to date the tethers are installed by lowering them to the ocean floor through the columns of the TLP itself. To enable this, the tethers are made up of threaded tubular segments which are secured together section by section as the tether is lowered. This arrangement presents a number of problems. The TLP must be provided with heavy hoisting equipment to support the great weight of the tether as it is lowered or raised. Additionally, the entire length of the columns of the TLP must be reserved for the tethers which pass therethrough. This space could otherwise be used for other purposes, such as housing drilling and production equipment. Further, through column tether installation is very time consuming. This increases the vulnerability of the TLP to adverse weather during the installation process.
It would be desirable to develop a TLP which avoids the need for through column tether installation.